1. Field of the Invention
This invention concerns the enhanced solubility of calcium phosphate fertilizer compositions and in particular calcium phosphate compositions useful for preparing aqueous fertilizer solution for precision plant nutrition.
2. Prior Art
In greenhouses, nurseries, and other intensive horticulture environments, best results are attained when macro- and micro-nutrients are carefully delivered to the growing plants. Many growers choose to utilize compound high analysis water soluble fertilizers. Typically, these fertilizers are marketed as solids which are dissolved to prepare concentrated stock solutions which are then diluted into irrigation water by means of proportioners or injection devices.
The Grace-Sierra Horticultural Products Company markets a wide range of water-soluble solid compound fertilizer formulations under the trademark Peters Professional. These formulations are designed to dissolve quickly and completely with no precipitation. It is also desired that a fertilizer formulation have good long term stability as a stock solution so as not to form precipitates which clog proportioners and irrigation lines in used by commercial growers. This has led to a limitation with water soluble fertilizer formulation available heretofore.
Calcium is an important nutrient which is called for in many plant nutrition formulations, but calcium salts cannot be used together with the conventional phosphorus sources-monoammonium phosphate and diammonium phosphate. As the pH's which these phosphate sources provide are to high (not acid enough) to give adequate solubility to the calcium ions and thus give rise to precipitation of the calcium salts in the stock solution and in the proportioners. Potassium phosphate or sodium phosphate can be used as soluble phosphorus sources but these are either expensive or not conducive to plant growth. Phosphoric acid can be used but is a liquid and hazardous to handle. Therefore, a grower wishing to fertilize with both calcium and phosphorus, without resorting to the use of an alkali metal phosphate or liquid phosphorus acid, will need to inject these two elements separately.
An alternative covered under U.S. Pat. No. 5,171,349, which is assigned to the same assignee as that of the present invention, and whose entire disclosure is incorporated by reference herein, is to utilize fertilizers which include urea phosphate as the sole phosphate source in order to maintain solubility of calcium such as calcium nitrate or calcium chloride. The low solubility of calcium phosphate either as monocalcium phosphate, dicalcium phosphate or tricalcium phosphate in aqueous solutions has precluded its use in providing adequate levels of calcium and phosphate. These materials are generally utilized as conditioning agents to reduce caking in fertilizer and food related compounds such as table salt.
In U.K. Patent No. 2,072,644 (Wolstein et al.) a procedure is disclosed for producing a nitrogenous fertilizer by heating in the range of 100.degree. to 130.degree. C. The process utilizes urea and phosphoric acid or urea phosphate or a mixture thereof added to a fertilizer feedstock comprising calcium nitrate.
The present invention is much simpler and does not require the addition of heat as in Wolstein.
The present invention teaches that solubility is attained with a mixture of calcium phosphate whether in the form of monocalcium phosphate, dicalcium phosphate and/or tricalcium phosphate with urea phosphate to form an aqueous solution.
The present invention employs urea phosphate in water soluble solid fertilizer formulations. Urea phosphate is a known material. The text, urea and phosphate fertilizers by Clifford W. Fowler (Noyes Data Corporation, Park Ridge, N.J. 1976) at page 23, shows a process for preparing this material and suggests that it may be used as a fertilizer. Similarly, U.S. Pat. No. 4,456,463 discloses the use of urea phosphoric acid reaction products as ammonia stabilized liquid components of fertilizer. Chemical abstracts 106(3) 101311Q shows a liquid component for use in fertilizers which contains a urea phosphate poly-condensation product. Chemical abstracts 102(9) 7748V also shows this.
Other references of interest include chemical abstracts 101(23) 209498S which concerns the biological fate of urea/phosphate fertilizers; chemical abstracts 101(21) 190398K which describes the use of urea phosphate fertilizers; chemical abstracts 88(19) 135558a which describes the reaction of that phosphoric acid reacts with urea and calcium salts; and chemical abstracts 86(11) 7073g which deals with the development of liquid multi-component fertilizers based on potassium and ammonium nitrates and diphosphate, urea, water and trace elements.